Using Compact Tokamaks to Accelerate the Development of Fusion Power

Is it possible to rapidly develop tokamak fusion facilities—devices that are most extensively used for harvesting the fusion reactions powering the sun and stars on Earth—to generate clean, safe, and almost limitless energy for producing electricity?

Physicist Jon Menard from the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) has analyzed this question while having a detailed observation at the concept of a compact tokamak provided with high-temperature superconducting (HTS) magnets. Magnets such as these have the ability to generate higher magnetic fields—required to initiate and sustain fusion reactions—which would otherwise not be feasible in a compact facility.

Menard first exhibited the paper, published now in Philosophical Transactions of the Royal Society A, at a Royal Society workshop in London that investigated speeding up of the development of tokamak-produced fusion power with compact tokamaks.

This is the first paper that quantitatively documents how the new superconductors can interplay with the high pressure that compact tokamaks produce to influence how tokamaks are optimized in the future. What we tried to develop were some simple models that capture important aspects of an integrated design.

Jon Menard, Physicist, Princeton Plasma Physics Laboratory

“Very Significant” Findings

According to Steve Cowley, director of PPPL, the outcomes are “very significant.” Cowley stated that “Jon’s arguments in this and the previous paper have been very influential in the recent National Academies of Sciences report,” calling for a U.S. program to create a compact fusion pilot plant to produce electricity at the lowest possible cost. “Jon has really outlined the technical aspects for much smaller tokamaks using high-temperature magnets,” noted Cowley.

Compact tokamaks, which can incorporate spherical facilities like the National Spherical Torus Experiment-Upgrade (NSTX-U) that is currently under repair at PPPL and the Mega Ampere Spherical Tokamak (MAST) in Britain, offer certain beneficial features. The devices, with cored-apple shapes and not doughnut-like traditional tokamak shapes, can offer high-pressure plasmas that are vital for fusion reactions with comparatively low and cost-effective magnetic fields.

Reactions such as these fuse light elements in the form of plasma—the hot, charged state of matter made of atomic nuclei and free electrons—to liberate energy. Researchers make attempts to reproduce this process and typically create a star on Earth to produce ample electricity for farms, homes, and industries across the globe. Fusion has the ability to last millions of years with reduced risk and without producing greenhouse gases.

Extends Previous Examination

Menard’s analysis extends his earlier examination of a spherical design with the potential to create components and materials for a fusion reactor and act as a pilot plant for electric power generation. This study offers a comprehensive analysis of the complex tradeoffs that future experiments will have to investigate with regards to integration of compact tokamaks with HTS magnets.

We realize that there’s no single innovation that can be counted on to lead to some breakthrough for making devices more compact or economical. You have to look at an entire integrated system to know if you are getting benefits from higher magnetic fields.

Jon Menard, Physicist, Princeton Plasma Physics Laboratory

The focus of the study is on key problems such as the size of the hole, called the “aspect ratio,” at the center of the tokamak that holds and shapes the plasma. In the case of spherical tokamaks, this hole could be half the size of that in traditional tokamaks, in proportion to the cored apple-like shape of the compact design. Although physicists consider that plasma confinement and plasma stability can be enhanced by lower aspect ratios, “we won’t know on the confinement side until we run experiments on the NSTX-U and the MAST upgrades,” stated Menard.

Lower aspect ratios are an ideal setting for HTS magnets, the high current density of which can generate the strong magnetic fields required for fusion to occur within the comparatively narrow space of a compact tokamak. Yet, superconducting magnets mandate thick shielding for protection from heating and damage caused by neutron bombardment, offering very less room for a transformer to induce current in the plasma to complete the twisting field upon reducing the size of the device. Consequently, for designs of lower aspect ratio, researchers would have to devise new methods to create some or all of the initial plasma current.

Electric Power of 200–300 Megawatts

In order to produce the 200–300 MW of electric power required to sustain the plasma, analyzed by the study, higher confinement than standard tokamak operating regimes would be required. Power generation such as this could result in challenging fluxes of fusion neutrons that would restrict the predicted lifetime of the HTS magnets to one to two years of full-power operation. Lifetime could be considerably increased by thicker shielding but it would also reduce the fusion power delivery.

Indeed, major advancement will be required for HTS magnets, which have not yet been developed to scale.

It will probably take years to put together a model of the essential elements of magnet size requirements and related factors as a function of aspect ratio.

Jon Menard, Physicist, Princeton Plasma Physics Laboratory

According to Menard, the crux is that the lower aspect ratio “is really worth investigating based on these results.” He noted that the possible advantages of lower ratios include the synthesis of fusion power density—the critical output of fusion power per volume of plasma—that surpasses the output for traditional aspect ratios. “Fusion needs to become more attractive,” stated Menard, “so it’s important to assess the benefits of lower aspect ratios and what the tradeoffs are.”

This study was supported by the DOE Office of Science.